1. Field of the Invention
The present invention relates to a method for producing an encapsulated toner for heat-and-pressure fixing used for development of electrostatic latent images in electrophotography, electrostatic printing, or electrostatic recording, and to an encapsulated toner obtained by the above method.
2. Discussion of the Related Art
As described in U.S. Pat. Nos. 2,297,691 and 2,357,809 and other publications, conventional electrophotography comprises the steps of forming an electrostatic latent image by evenly charging a photoconductive insulating layer, subsequently exposing the layer to eliminate the charge on the exposed portion and visualizing the formed image by adhering colored charged fine powder, known as a toner, to the latent image (a developing process); transferring the obtained visible image to an image-receiving sheet such as a transfer paper (a transfer process); and permanently fixing the transferred image by heating, pressure application or other appropriate means of fixing (a fixing process).
As indicated above, the toner must meet the requirements not only of the development process, but also of the transfer process and the fixing process.
Generally, a toner undergoes mechanical frictional forces due to shear force and impact force during the mechanical operation in a developer device, and deteriorates after copying from several thousands to several ten thousands of sheets. The deterioration of the toner can be prevented by using a tough resin having such a high molecular weight that it can withstand the above mechanical frictional forces. However, this kind of a resin generally has such a high softening point that the resulting toner cannot be sufficiently fixed by a non-contact method, such as oven fixing, because of its poor thermal efficiency. Further, when the toner is fixed by a contact fixing method, such as a heat-and-pressure fixing method using a heat roller, which is excellent in thermal efficiency and therefore widely used, it becomes necessary to raise the temperature of the heat roller in order to achieve sufficient fixing of the toner, which brings about such disadvantages as deterioration of the fixing device and curling of the paper. Furthermore, the resin described above is poor in grindability, thereby remarkably lowering the production efficiency of the toner. Accordingly, the binder resin having too high of a degree of polymerization and also too high of a softening point cannot be used.
Meanwhile, according to the heat-and-pressure fixing method using a heat roller, the thermal efficiency is excellent, so that this method is widely used in various high-speed and low-speed copy machines. However, when the surface of a heat roller contacts the surface of the visible image, the toner is likely to cause a so-called "offset phenomenon," wherein the toner is adhered to the surface of the heat roller, and thus transferred to a subsequent transfer paper. In order to prevent this phenomenon, the surface of a heat roller is coated with a material having excellent release properties for the toner, and further a releasing agent such as a silicone oil is applied thereon. However, the method of applying a releasing agent is likely to bring about various problems such as high costs and device troubles.
Although processes for improving the offset phenomenon by unsymmetrizing or crosslinking the resins have been disclosed in Japanese Patent Examined Publication No. 57-493 and Japanese Patent Laid-Open Nos. 50-44836 and 57-37353, the fixing temperature has not yet been improved by these processes.
Since the lowest fixing temperature of a toner is generally between the temperature of low-temperature offsetting of the toner and the temperature of the high-temperature offsetting thereof, the serviceable temperature range of the toner is from the lowest fixing temperature to the temperature for high-temperature offsetting. Accordingly, by lowering the lowest fixing temperature as much as possible and raising the temperature at which high-temperature offsetting occurs as much as possible, the serviceable fixing temperature can be lowered and the serviceable temperature range can be widened, which enables energy saving, high-speed fixing and prevention of curling of paper.
From the above reasons, the development of a toner having excellent fixing ability and offset resistance has always been desired.
A method has been proposed to achieve low-temperature fixing by using an encapsulated toner comprising a core material and a shell formed thereon so as to cover the surface of the core material.
Among such toners, those having a core material made of a low-melting wax which is easily plastically deformable, as described in U.S. Pat. No. 3,269,626, Japanese Patent Examined Publication Nos. 46-15876 and 44-9880, and Japanese Patent Laid-Open Nos. 48-75032 and 48-75033, are poor in fixing strength, so that they can be used only in limited areas, although they can be fixed only by pressure. Further, in the case where toners having a liquid core material are used, the shell materials tend to break in the developer device and stain the inside thereof. Thus, it has been difficult to control the strength of the shell materials.
Therefore, as a toner for heat-and-pressure fixing, an encapsulated toner for heat roller fixing has been proposed, which comprises a core material made of a resin having a low glass transition temperature which serves to improve the fixing strength, though blocking at a high temperature may take place if used alone, and a shell made of a high-melting point resin wall which is formed by interfacial polymerization for the purpose of imparting a blocking resistance to the toner.
Such encapsulated toners are disclosed in Japanese Patent Laid-Open No. 61-56352, and encapsulated toners with further improvements have been proposed (see Japanese Patent Laid-Open Nos. 58-205162, 58-205163, 63-128357, 63-128358, 63-128359, 63-128360, 63-128361, and 63-128362). However, since these toners are prepared by a spray drying method, the equipments for the production thereof become complicated. In addition, they cannot fully exhibit the performance of the core material, because they have not come up with a solution for the problems by the shell material.
Therefore, an encapsulated toner using a compound having thermal dissociation property as a shell material (Japanese Patent Laid-Open No. 4-212169) and an encapsulated toner using an amorphous polyester as a shell material have been proposed (Japanese Patent Laid-Open No. 6-130713). In cases of producing the encapsulated toners mentioned above, from the viewpoint of simplifying the production process and the production facilities, the above encapsulated toners are advantageously produced by a process comprising the steps of suspending polymerizable monomers in a dispersion medium, and forming a shell by an interfacial polymerization or in situ polymerization.
On the other hand, the following additives are conventionally added in suitable amounts to the core material of the encapsulated toner. Conductive materials are added for improving cleanability and stabilizing triboelectric charges; charge control agents are added for controlling triboelectric charges to positive or negative polarity; wax components are added for improving offset resistance; color pigments are added for coloring; and particulate magnetic materials are added for magnetizing the toner.
The additives mentioned above are generally solids, which are mostly insoluble in the polymerizable monomers. Also, as for additives, such as charge control agents and color pigments, the additives are normally present in the form of aggregates of particles. Therefore, in the case of producing toners by suspension polymerization, toners are produced by a process comprising the steps of adding the above additives to the polymerizable monomers, sufficiently disintegrating in advance the aggregated particles using mixers such as a ball mill and a sand stirrer to disperse the particles into the polymerizable monomers; and polymerizing the monomers.
The additives, such as the charge control agents added for stabilizing triboelectric charges and the conductive materials added for improving cleanability, can exhibit excellent effects when the additives are present in the vicinity of the toner surface. However, when the additives are dispersed by the dispersion method as mentioned above, the additives are likely to be incorporated into the inner portion of the toner, so that few additives are present on the toner surface. Therefore, advantageous effects by adding the additives cannot be obtained.
In order to solve the problems, Japanese Patent Laid-Open Nos. 1-185652, 1-185659, and 1-185665 disclose methods for producing toners comprising the step of adding an additive or fine resin particles containing an additive to the toner obtained by suspension polymerization to fix the additive components on the toner surface. By these methods, the additives can be present on the surface of the toner to fully exhibit their functions. However, in these methods, the production facilities are costly, and the dispersion of the additives externally added on the toner surface is poor, and thereby the production stability of the toner becomes poor. Also, since not all of the additives are strongly fixed to the toner surface, insufficiently fixed additives may become detached upon printing, and thereby the inside of the machine is stained.